The stainless steel thermos, that loyal companion of the winter hillwalker, suffers from a wee problem: whether it’s tea or coffee that is your poison, a dark coating will build up in the thermos over time. This stain stubbornly resists scrubbing with a brush, not helped by the limited access. Does that mean we are for ever doomed to the taste of stale tea and coffee with whatever we put in there? As it happens there is a simple solution (and, no, it’s not a new thermos).

The answer is soda crystals. Not bicarbonate soda, but rather sodium carbonate, the stuff that you can buy in kilo bags from Tescos for cleaning floors and what not. Simply put some into the flask (about a table spoon for 3/4l) add boiling water, stir with something long an thin, and watch years of deposit peal off like burnt skin, and the thermos look inside like brand new (if it’s every stubborn, repeat, maybe shuggle a bit with a brush). Rinse thoroughtly, obviously.

You can use this on anything that is made of stainless steel, glass or plastic (but not aluminium), works well for cleaning stainless steel coffee filter when it gets clogged up, which is what reminded me I have been meaning to mention this for years. Just make sure to rinse it well before using it again.